BELLINGHAM, Wash. -- The three men who lived in the light-green house on Northwest Avenue hardly ever spoke to neighbors, but the neighborhood knew all about them: their names, backgrounds and crimes.

The men were registered sex offenders.

Police had notified residents when the first of them moved into the quiet Columbia neighborhood, north of downtown, about three years ago. Schools handed out fliers warning students of their presence.

Late Aug. 26, two of the men were shot to death in their home while the third was away at work. Police say the killer, still at large, had knocked on their door claiming to be an FBI agent warning of an Internet "hit list."

On Wednesday, the local newspaper received a letter from someone claiming responsibility for the slayings and threatening to kill all Whatcom County sex offenders designated as Level III, considered the most likely to commit similar crimes again.

Now, as Bellingham police investigate what appears to be a case of vigilantism, local leaders and activists have renewed the debate over the 1990 state law requiring sex offenders to register their addresses.

The victims' addresses had been posted on the city's Web site.

"If this is a case of revenge or vigilantism, then it brings to light the question, 'Are there unintended consequences of this well-intentioned law?' " said Bellingham Mayor Mark Asmundson, an attorney.

Washington was the first state to pass such a law, which is intended to help the public keep track of dangerous sexual predators. In 1994, Congress mandated states to create registers of sex offenders. Now all 50 states have their own version of Washington's "Community Protection Act."

Such monitoring and public notification has made it difficult for many sex offenders to find a place to live once released from prison. In one highly publicized local case in 1993, a sex offender notified authorities of his plan to move into a family home south of here, in Snohomish County. Before he could move in, someone burned the house to the ground.

Many databases provide the general location of sex offenders, but Bellingham and Whatcom county Web sites provide exact addresses with photos of the offenders and descriptions of their crimes.

Whatcom County, a mountainous, mostly rural region about 75 miles north of Seattle, has 31 registered Level III offenders. Bellingham, the county seat with a population of 71,000, had six Level III offenders.

Among them were Hank Adolf Eisses, 49, convicted of child rape; Victor Manuel Vasquez, 68, convicted of child rape and molestation; and James Russell, 42, released a month ago after serving time for child molestation.

It was a quiet Friday night about 9 p.m. when a white man in his late 40s and wearing a blue jumpsuit and black baseball cap with an FBI logo knocked on the door of the house, according to police.

The man said he was there to warn the three about the hit list. Soon after, with the man still at the house, Russell left for work. When he returned about 3 a.m., he found Eisses and Vasquez dead of gunshot wounds.

Police have discounted Russell as a suspect because it was confirmed that he was at work during the estimated time of death. Neighbors also reported seeing the man with the FBI cap at the house.

Both Eisses and Vasquez had committed their crimes in Whatcom County, and some city employees speculated that one motive could be revenge rather than random vigilantism. Police have refused to disclose more details of the case.

"We haven't ruled out any motive," said Lt. Craige Ambrose. At the same time, he said the department was warning all local Level III sex offenders of the death threat relayed by the Bellingham Herald newspaper.

Earlier in the week, Police Chief Randall Carroll told the Seattle Times that, "If sex offenders were targeted and attacked because of their offense, the Legislature could decide they could repeal our sex-offender notification law."

Retired law professor John Q. La Fond said that would be the correct course of action. La Fond, who was on the faculty at the University of Missouri-Kansas City, argued on behalf of the American Civil Liberties Union against the state's notification law. He said public notification virtually "invites society to take the law into their own hands."

La Fond said research shows that public notification laws do not prevent sexual violence nor make sexual crimes easier to solve. He called the laws "symbolic but futile gestures" made by a society groping for a way to deal with a complex problem.

State Sen. Dale Brandland, a Republican from Whatcom County and a former county sheriff, said there's virtually no possibility that Washington's notification law will be repealed as a result of the murders. Given the climate of anxiety regarding sexual predators, he said, the Legislature might make notification laws more stringent.

Brandland referred to the recent case of Joseph Edward Duncan III, a 42-year-old convicted sex predator from Tacoma, Wash., who is being investigated in the deaths of six children and two adults across four states. Duncan is being held in Kootenai County, Idaho, for the murders of four people and the kidnapping of two young children in May near Coeur D'Alene, Idaho.

"People here were outraged that he was loose and that terrible crime was allowed to happen," Brandland said.

"If there was any kind of leaning by the public, it would be to make the law stricter and to make it even tighter for sex predators. There's very little sympathy for people who prey upon defenseless children."

Asmundson, the mayor, said he has friends in Bellingham who've told him, "Too bad they didn't get the third one."

There's been a trickle of sympathy for Eisses and Vasquez. Although the city has held no public memorials for the slain sex offenders, some people have dropped off bouquets of flowers and handwritten cards at the front gate of their house. On one blank sheet of paper next to a wilting orchid, someone had scrawled the words, "It is wrong to kill. The end."